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Anti-Xa
Test CodeANTXA
Alias/See Also
Heparin Level, UFH, LMWH, Xa
Preferred Specimen
2.7mL (9:1 blood to anticoagulant) in a 2.7mL light blue top tube
Minimum Volume
1.8mL (9:1 blood to anticoagulant) in a 1.8mL light blue top tube
Instructions
Tube must be filled to the line to ensure the proper ratio of blood to anticoagulant.
Transport Container
Light blue top tube (3.2% sodium citrate)
Specimen Stability
Room temperature: 2 hours, Refrigerated: not recommended, Frozen platelet-poor plasma: 1 month
Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)
Moderate or grossly hemolyzed specimens are unacceptable.
Limitations
Anti-Xa assay underestimates heparin concentration in the presence of significant AT deficiency, pregnancy, end stage renal disease, and post-thrombolysis and in patients with hyperbilirubinemia.
Likewise, patients with polycythemia will have overestimated levels of anticoagulation due to a lower plasma-to-citrate ratio compared with patients with hematocrit values within a normal range
Likewise, patients with polycythemia will have overestimated levels of anticoagulation due to a lower plasma-to-citrate ratio compared with patients with hematocrit values within a normal range
Reference Range
UFH Therapeutic Range: 0.3 - 0.7 Intl Units/mL
LMWH Therapeutic Ranges (target anti-Xa levels measured 4-6 hours after dosing):
Enoxaparin (Lovenox)
Therapeutic bid dosing 0.5 - 1 Intl Units/mL
(1 mg/kg/dose BID or daily due to renal adjustment)
Therapeutic daily dosing 1 - 2 Intl Units/mL
(1.5 mg/kg/dose daily)
Prophylactic 0.2 - 0.5 Intl Units/mL
LMWH Therapeutic Ranges (target anti-Xa levels measured 4-6 hours after dosing):
Enoxaparin (Lovenox)
Therapeutic bid dosing 0.5 - 1 Intl Units/mL
(1 mg/kg/dose BID or daily due to renal adjustment)
Therapeutic daily dosing 1 - 2 Intl Units/mL
(1.5 mg/kg/dose daily)
Prophylactic 0.2 - 0.5 Intl Units/mL
Clinical Significance
Unfractionated heparin therapy is the drug of choice when a rapid anticoagulant effect is needed. The response to unfractionated heparin therapy varies widely between individuals and therefore laboratory monitoring is absolutely required. Studies have indicated that the best response occurs when an individual is maintained between 0.3 – 0.7 IU/mL. Laboratory monitoring of UFH has traditionally been performed with the APTT. However, this is a nonspecific test for Heparin activity. APTT clotting times are affected by factor deficiencies, circulating anticoagulants and increased levels of Fibrinogen or Factor VIII. The Heparin assay is more specific and is not affected by these factors and is thus the preferred method of monitoring.
In contrast to UFH which is administered in an emergency situation, LMWH therapy is often prescribed for patients requiring long-term anticoagulant therapy. As a result of the manufacturing process, LMWH preparations exhibit higher anti-Xa activity than anti-thrombin activity and thus have a much more predictable dose response. Monitoring of LMWH is only needed in certain situations such as during pregnancy, in pediatric patients, obese patients or in the elderly population.
In contrast to UFH which is administered in an emergency situation, LMWH therapy is often prescribed for patients requiring long-term anticoagulant therapy. As a result of the manufacturing process, LMWH preparations exhibit higher anti-Xa activity than anti-thrombin activity and thus have a much more predictable dose response. Monitoring of LMWH is only needed in certain situations such as during pregnancy, in pediatric patients, obese patients or in the elderly population.
